1. Field of the Invention
The present invention relates to a catalyst element for purifying exhaust gases from an internal combustion engine. It relates, in particular, to a catalyst element for purifying exhaust gases (hereinafter also simply called a catalyst element) which is superior in heat resisting property and in resistance against vibrations, and which is capable of performing superior purifying characteristics even under severe thermal conditions and vibrating conditions.
2. Description of the Related Art
In a motorcycle and a four-wheeled vehicle, a catalyst element is disposed in an flow passage of exhaust gases in order to reduce the contents of carbon monoxide (CO), hydrocarbons (HC), or the like in the exhaust gases from an internal combustion engine thereof.
As a catalyst element, there is generally known a three-way catalyst which is made by coating a metallic honeycomb-shaped carrier (or substrate) made, for example, of a heat-resistant stainless steel with a catalytic substance containing precious metals. As a catalyst element for use in a two-wheeled vehicle such as a motorcycle, there is known one in which an exhaust pipe is directly coated on its internal surface with a catalytic substance (see Japanese Published Unexamined (i.e., publicly laid-open) Patent Application No. 8912/1975) or one in which a catalytic substance is carried on (or contained in) a fabric carrier (or substrate) made of knitting or weaving a fibrous heat-resistant inorganic oxide (see Japanese Published Unexamined Patent Application No. 117816/1979).
Among the above-described catalyst elements, in the catalyst element in which the catalytic substance is directly carried on the metallic carrier, the catalytic substance is likely to be peeled off the carrier due to a thermal expansion or vibrations of the base material (e.g., stainless steel) which constitutes the carrier. In the catalyst element in which the carrier is made of a fabric material, on the other hand, the catalytic substance is likely to be peeled off the carrier due to vibrations and due to wear that is caused by the vibrations. Both of the above-described catalyst elements have therefore a disadvantage in that the purifying capacity will consequently be decreased. They have both a disadvantage in that the durability is poor and therefore that a sufficient exhaust gas purifying capacity will cease to be obtained when used in a vehicle, particularly in a motorcycle, for services under severe conditions such as repeated heating and cooling as well as vibrations.
In order to eliminate this kind of disadvantages by increasing the adhering strength between the metallic carrier and the catalytic substance, there is known a catalyst element in which a metallic carrier is coated on its surface with alumina by means of plasma spraying, whereby the catalytic substance is carried on the plasma-sprayed layer (see Japanese Published Unexamined Patent Application No. 85318/1991). In case a hollow carrier, for example, is used in this prior art, it is not possible to sufficiently and uniformly spray the inner peripheral surface of the carrier with alumina by plasma spraying. It has therefore a problem in that there is a limitation or restriction to the shape of the carrier that can be employed in carrying out this prior art.
Further, there is also known a catalyst element in which a catalytic substance is adhered to the surface of a ferritic stainless steel containing aluminum which has formed on its surface alumina whiskers by heat treatment at a high temperature (see Japanese Published Unexamined Patent Application No. 157143/1991). However, in order to obtain a good shape of alumina whiskers, there is a limitation or restriction to the material that can be used. Any one of the above-described catalyst elements is expensive for use as a catalyst element for purifying exhaust gases in an vehicle, especially a motorcycle, and is therefore not suitable for practical use.
The present invention has an object of solving the above-described.problems by providing a catalyst element for purifying exhaust gases from an internal combustion engine, in which a catalytic substance is carried on or held in a metallic carrier at a high strength of adhesion and which performs high purifying characteristics under severe conditions of high temperature and vibrations such as are encountered in a motorcycle or a four-wheeled vehicle.
In order to attain the above and other objects, the present invention is a catalyst element for purifying exhaust gases having a metallic carrier and a catalyst carrying layer, characterized in that a coating layer of a heat resistant inorganic oxide is formed on a surface of the metallic carrier; and that the catalyst carrying layer is formed on the coating layer.
The heat resistant inorganic oxide is superior in the characteristics of adhesion to metal and functions as an adhesive agent to adhere the catalyst carrying layer to the metallic carrier. The catalyst carrying layer can thus be adhered to the carrier at a high peeling strength (i.e., a high strength of resistance to peeling off). As a result, the amount of peeling of the catalytic substance in the catalyst carrying layer when the carrier is subjected to thermal expansion and vibrations can be minimized.
In addition, by forming the coating layer of heat-resistant inorganic oxide, the heat resisting property of the carrier is improved and also the carrier is protected against strong acid or chloride ion. Therefore, the resisting property against corrosion and acid is improved, resulting in an improvement in the durability.
The metallic carrier may be made in various shapes such as a plate shape, a tube shape, or a honeycomb shape. However, if it is made of a heat-resistant stainless steel thin plate with perforations, the heat resisting property is improved. Further, since perforations are formed by punching the stainless steel plate, a larger surface area can be obtained and thus the characteristics of purifying the exhaust gases are improved. In addition, since the exhaust resistance inside the exhaust pipe becomes small, the catalyst element is suitable for use in a motorcycle or a motor vehicle.
Among the heat-resistant inorganic oxides, alumina, silica, aluminum silicate and alkali metal silicate have a high strength of adhesion to a metallic surface. Therefore, if the heat resistant inorganic oxide is one or more selected from the group consisting of alumina, silica, aluminum silicate, and alkali metal silicate, a high adhesive strength can be obtained. It also functions well as an adhesive agent between the carrier and the catalyst carrying layer.
Further, the catalyst carrying layer preferably comprises: one or more of precious metals of platinum (Pt), palladium (Pd), and rhodium (Rh); and a metallic oxide containing activating alumina as its chief component.
Still furthermore, a total amount of the precious metal or metals carried on the catalyst carrying layer is preferably 2 g or more per unit surface area of 1 m2 of the catalyst carrying layer, and a ratio by weight of the precious metals, where platinum and rhodium are carried, is preferably 20 parts of platinum (Pt) to 1 through 4 parts of rhodium (Rh).
In addition, a particularly preferred catalyst element for purifying exhaust gases comprises a metallic carrier and a catalyst carrying layer, and a coating layer of a heat resistant inorganic oxide formed on a surface of said metallic carrier; wherein said metallic carrier is a cylindrical member with a plurality of perforations, said catalyst carrying layer is formed on said coating layer; and said heat resistant inorganic oxide includes alkali metal silicate and at least one selected from the group consisting of alumina, silica and aluminum silicate, said heat resistant inorganic oxide including sodium oxide (Na2O) from 0.7 to 4.5 percent by weight.